The membrane touch switch is a normally open, low voltage, pressure-sensitive device currently being used in a wide variety of applications, including appliances, electronic games, keyboards and instrumentation. It is usually fabricated as a three-layer sandwich with the conductive traces printed on the inner sides of the top and bottom layers which are separated by a spacer sheet. Pressure applied to the top layer establishes momentary electrical contact between the top and bottom layers through punched openings in the spacer sheet. Both flexible and rigid switches are available. The former are typically printed over a flexible polyester base, while the latter use a printed circuit board bottom panel.
Ease of design and manufacture allow touch switches to cost less than their electromechanical counterparts. Nevertheless, it is still imperative that they be made from high reliability electronic materials and that these materials be compatible with each other. Since the high cure temperatures of the many inks available for cermet applications are not suitable for polymeric substrates, many polymer thick film conductors and dielectrics have been developed for this application. A variety of chemistries is currently used for both types of inks, and a variety of processing options are used as well.
In practice, most manufacturers first select a conductive ink, then look for a compatible dielectric. The selection is especially critical in this application since the dielectric is used both to insulate the conductor, to allow crossovers, and to encapsulate it to prevent environmental damage. However, lack of adequate adhesion of the dielectric to the substrate and/or to the conductive ink has resulted in limited market penetration for many dielectric compositions, especially those which are UV curable.
Existing manufacturing processes dictate that the dielectric be screen-printable and either thermally curable or UV light curable. Faster cures which can be obtained with the latter make it the more cost-effective approach and the wide availability of UV curing units makes this a practical route. The dielectric must be compatible with the conductive ink and must meet certain performance standards. It must cure to a flexible, abrasion-resistant film, free of pinholes with good adhesion to the substrate and to the conductive ink. Crossover applications also require that the conductive ink have good adhesion to the dielectric and, frequently, good adhesion of the dielectric to itself is also specified. Electrical requirements call for a low dielectric constant, high insulation resistance and high breakdown voltage. The physical and electrical properties must not degrade under a variety of environmental conditions.
In an assembled switch, dielectric failure can lead to either electrical or physical breakdown of the switch. Both materials vendors and switch manufacturers rigorously test components under fresh and accelerated aging conditions to decrease the probability of this occurrence. Electrical failure implies that shorting has occurred because of pinholes, the presence of conductive impurities in the formulation, dielectric failure under load, or other stressful environmental conditions. Physical failures originate from blistering, softening or cracking, any of which can occur during the manufacturing process or during use. Blistering may be due to incompatibility of the dielectric with the conductor or the substrate, as well as from moisture susceptibility. Softening can occur under high humidity conditions or with solvent from a conductor ink, and cracking can result from the inherent brittleness of a cured composition. All of these problems can be prevented with appropriately formulated inks.
A more difficult problem is that of adhesive loss and since this is intimately related to the substrate, the problem is compounded by the large number of available substrates. While polyester films are the most widely used in touch switches, polycarbonate and polyimide films are occasionally encountered. Each film manufacturer typically offers several grades of each product, with different surface characteristics due to variable processing techniques and/or surface pretreatments. The films may also be given a heat treatment to reduce shrinkage in later curing steps.
Different polyester films have different physical surfaces. For example, both Mylar.RTM. EL 500 and 500D(.sup.7) polyester films show evidence of rough surfaces due to to slip pretreatment to allow easy handling of these films, while Melinex.RTM. 0(.sup.6) polyester film has an extremely smooth surface. The Mylar.RTM. 500D polyester film has much smaller particulates than the Mylar.RTM. EL 500 polyester film, giving it a clear appearance while the Mylar.RTM. EL 500 has a cloudy appearance. The Melinex.RTM. 0 polyester film is also very clear but suffers from poor handleability and tends to stick to itself. As might be predicted, adhesion to these surfaces is quite variable and indirectly related to surface smoothness--the Melinex.RTM. 0 polyester film generally giving the poorest values. Since membrane switch manufacturers often select their substrates not for microscopic structure but for reasons of cost, dimensional stability and visual appearance, the physical surface characteristics are frequently overlooked, yet may be critical to the performance of an ink from the standpoint of adhesion.